Cochlear implants are electronic medical device to help deaf or severely-hearing-impaired people. They typically consist of an external signal processor, a transmission coil, an implantable package with a receiver coil, a hermetically sealed circuit, and an electrode array. More particularly, these systems include a microphone to receive sounds and convert them into corresponding electrical signals. These electrical signals may then be processed to generate a series of stimulation pulses that are delivered to the inner ear using a series of implanted electrodes. The stimulation of these implanted electrodes allows the implantee to perceive the corresponding ambient sounds.
A typical cochlear implant includes both an external component and an internal component. The external component will typically include a microphone, a speech processor, and a radio-frequency transmitter, while the internal component includes an implanted receiver, a hermetically-sealed decoding circuit, and a series of implanted electrodes. However, there are also numerous other designs currently available. Regardless of the specific configuration, a basic premise of all cochlear implant devices is that ambient sounds are detected by the microphone and a transduced signal representative of this signal is then generated. The transduced signal is then processed by a speech processor in accordance with one of several possible strategies.
One of the primary design considerations for cochlear implants is the current source design. To that end, there are two primary types of current source designs currently in use. The first is to use one current source for all N electrodes, while the second approach is to use N current sources for N electrodes. Some products even use 2N current sources for N electrodes for more flexibility. Each solution has its own advantages and disadvantages. For example, with one current source for all electrodes, the size, complexity, and power consumption of current stimulator are low. But the stimulation mode is also restricted by the ability of one current source. No simultaneous stimulation, current steering, or multi-polar stimulation strategies are supported. For N (or 2N) current sources for N electrodes, more flexibility and functionality of stimulation are achieved at the expense of size, complexity, and power consumption.
Current resolution is an important factor for current source design, especially for low stimulation level. For cochlear implant users, the ratio of current variation versus current ΔI/I is more important than the current variation ΔI itself. Traditional linear step-size current source uses a constant ΔI. Therefore, at low stimulation level where I is small, ΔI/I is large. To lower ΔI/I, one solution is to increase the number of current amplitude bit. However, this results in an increase in the number of current sources in the internal circuit and lowers the stimulation rate (8-bit requires 256 unit current sources and 10-bit requires 1024 unit current sources). When the stimulation level is close to the most comfortable loudness (MCL) and the value I is large, ΔI/I is usually too small and the resolution space is wasted. As such, there is a need for an improved cochlear implant which provides a more balanced solution for both complexity and functionality.